In a typical power-distribution application, power from a central source is distributed through branch circuits to a load device. The branch circuits are equipped with protection devices, such as circuit breakers or fuses. During an electrical fault, such as a short circuit, the protection devices detect an abnormally high level of current and disconnect or interrupt the source from the load before causing damage or fire to the distribution system.
The introduction of the ground fault interrupter (GFI) added electrocution protection to the distribution system by detecting an imbalance between phase currents in a particular branch circuit, indicating that current is flowing through an alternative ground path and possibly in the process of electrocuting an individual.
There are, however, significant shortcomings in traditional distribution protection methods. For example, a fire could still occur from a loose connection. In this case, the series resistance of a live connection increases and heats up to the point of igniting surrounding materials. Alternatively, at sufficiently high voltages, a conductive plasma may form across a small air gap.
The heat build-up from a series fault condition could occur at electrical currents well below the trip point of the branch circuit protection devices. In the case of GFI protection, the GFI circuit can only protect an individual that comes in contact with both a line conductor and a ground point, such as would be the case if an individual touched a live electric conductor with one hand and a sink faucet with the other hand. However, if the individual manages to touch both a live conductor and a return path (such as across the “hot” and neutral conductors of a home outlet) the GFI would not activate, and the person would receive a shock.
There are two primary fault modes to be detected. The first mode is an in-line or series fault where an abnormal resistance is put in series with the path between the source and load. The second fault mode is a cross-line or parallel fault, where, for example, a human or animal touches one or both exposed wires or contacts along the transmission line while it is transporting power.
The requirements for the maximum voltage, current and duration for human contact with an electrical distribution system delivering alternating-current (AC) or direct-current (DC) power are defined in the IEC 60479-1 standard.